This phrase is one of the cases where "were" is still required in the third person singular. In other counterfactual statements, we can use either "was" or "were", for instanceI wish I were/was going with you.If I were/was stronger...

"were" is also required when it is inverted:Were I stronger...*Was I stronger...

And also when followed by another verb:If I were to go...?If I was to go...